1. Field of the Invention
The present invention relates to an image-displaying apparatus, particularly suitable for a projector or an image-observing apparatus.
2. Related Art Statement
There are currently, commonly available two types of color display methods for projectors. One type is a color-mixing method in which light emitted from a white thermal light source is divided into three primary colors: red, green and blue, three light valves are illuminated with three color lights, respectively, the lights modulated by the respective light valves are combined with a color-combining optical system such as a cross dichroic prism, and an illumination light of the three primary colors modulated by the light valves is constantly illuminated upon each of pixels on a screen. The other is a time-shared color-mixing system in which the three primary colors are divided in terms of time and the divided colors are successively projected upon the screen. The former type is called a “three-plate system” because three light valves are used. The latter is called a “single-plate system” because one light valve has only to be used.
In order to downsize the projector, the single-plate system is more desirable from the standpoint of the cost, because the three-plate system requires a space for a color-separating optical system and the three light valves. On the other hand, a so-called thermal light source lamp such as a super high pressure mercury lamp or a xenon lamp is used for the single-plate system as well as the three-plate system in the case of the conventional projectors. Since the above thermal light source is used as an illuminating light source, an input electric power needs to be large, because the conversion efficiency from the input power to the light is low. Consequently, dimensions of the power source and the lamp (which is provided with a reflector mirror in many cases) become large, and a cooling fan is necessary, Thus, such a system is heavy, and the span of life is shortened because of heat in the case of the light valve, particularly the liquid crystal valve.
Under the circumstances, there are various proposals that light emitting diodes (hereinafter abbreviated as “LEDs”) which have been actively being developed are used as light sources for the projectors. The LED generally possesses advantages such as long span of life, high efficiency, high-speed response, unicolor light emission, etc. Further, since the LED further possesses rapidly increased brightness, such LED devices have been expected to be applied to many illuminating fields.
As the technique for using the LED in the projector, JP-A 2001-249400 is recited, for example. According to this publication, an array of light-emitting devices is used as a light source, each of the light-emitting devices inside the array illuminates the entire light-modulating area of the light valve (light modulator), and color displaying is carried out by turning on or off the light-emitting devices for the three primary colors at a high speed.
The above color displaying is possible in such a method with respect to reflection type liquid crystal display devices described in JP-A 2002-244211 and JP-A 2003-241148 and a display device, such as DMD, in which light-modulation information on a surface of a light valve is changed in a lump.
As the prior art, there are further JP-A 2004-146200 and Japanese Patent No. 3,257,646. A prism described in JP-A 2004-146200 is intended for a direct-view type organic EL display device. Because of the direct view type, this prism is aimed only at effectively taking out image-displaying light from each pixel in a front face direction. A prism in Japanese patent No. 3,257,646 is aimed only at branching laser beam of a light source.
However, the LED has a light intensity distribution, generally so-called “orientated light distribution”, in which intensities of light ordinarily differ from one another, depending upon emitting angles. In case of a bared chip type LED, the orientated light distribution is usually a so-called “Lambert” distribution.
FIG. 15 shows a orientated light distribution of an LED of the bared chip type. As shown in FIG. 15, the light intensity in a θ-direction varies according to cos θ in which θ is an angle from the center of the LED 100 in a light-emitting direction.
FIG. 16 shows the construction of an image-displaying apparatus using the LEDs having the orientated light distribution as shown in FIG. 15. When the entire light valve 102 is illuminated with a plurality of the LEDs 100 via a release lens 101, the orientated light distributions of the LEDs 100, 100 are interposed upon each other as shown in FIG. 16, so that uneven illuminance appears on the light valve 102.
Such a characteristic naturally appears in a so-called bombshell type LED formed when the LED is coupled with a lens.
In order to solve the problem on the above orientated light distribution and characteristic of the light source (LED), light flux from the light source is generally divided into plural ones by using a cable stitch lens unit.
However, it is not practical to couple the cable stitch lens unit to the LEDs. Even if the former is coupled to the latter, the size of each of lenses constituting the cable stitch lens unit becomes extremely small. Thus, if the entire surface of the light valve is to be illuminated with secondary light sources formed by the respective lenses inside the cable stitch lens unit at an a focal magnification of the relay lens system, the illuminating system becomes extremely large in the optical-axis direction.
If the light is led from each of the LEDs to the light valve as much as possible, the F-number of the release lens system for leading the light from the LEDs to the light valve becomes smaller, which results in the increased lens diameter and the increased number of the aberration-correcting lenses.